Delfyett et al. have disclosed a high-power, narrow-pulsewidth laser in "Generation of subpicosecond high-power optical pulses from a hybrid mode-locked semiconductor laser," Optics Letters, volume 15, 1990, pp. 1371-1373. This laser, whose essential elements are illustrated in FIG. 1, combines passive and active mode locking. An optical cavity is formed between a fully silvered mirror 10 and a partially silvered mirror 12. That is, the fully silvered mirror 10 is highly reflective while the partially silvered mirror 12 transmits a portion of the light as the laser output. The cavity length L is relatively long so that it supports a large number of optical modes having a frequency spacing of ##EQU1## where c is the speed of light taking into account any dielectric material within the cavity. A semiconductor optical amplifier or traveling wave amplifier 14 is disposed in the middle of the cavity and is driven by both a DC bias and an RF signal at frequency f.sub.RF coupled into the amplifier 14 through an inductor 16 and capacitor 18 respectively. A thin multiple quantum-well structure 20 affixed to the fully silvered mirror 10 serves as a saturable absorber, following the disclosure of Silberberg et al. in "Subpicosecond Pulses from a Mode-Locked Semiconductor Laser," IEEE Journal of Quantum Electronics, volume QE-22, 1986, pp. 759-761. A saturable absorber is an optical absorber that highly absorbs low-intensity light, but, above a threshold of light intensity, its absorption progressively decreases. A quantum well is a semiconductor layer sandwiched between two barrier layers and of such thinness that quantum levels are formed in the well and the absorption spectrum becomes dependent on the well thickness. All known structures having multiple quantum wells (MQW) have a periodic structure of multiple, equally thick wells separated by equally thick barriers. An MQM structure operates well as the saturable absorber because the MQW room-temperature excitonic resonances saturate at relatively low optical intensities.
Passive mode locking, to be described first, does not require the RF drive signal to be applied to the optical amplifier 14, which instead simply amplifies within its relatively wide gain bandwidth whatever spontaneous optical emission occurs within the cavity. However, only those optical modes supported by the cavity are resonantly amplified. Furthermore, the low-intensity non-resonant modes are preferentially absorbed by the saturable absorber 20. There results a substantial number of amplified resonant modes separated by the frequency spacing .DELTA.f. The saturable absorber 20 further causes these modes to be phase locked so as to maximize the instantaneous intensity of the total optical signal. By straightforward Fourier analysis, an optical signal having many frequency components with a frequency spacing .DELTA.f has a temporal form of a pulse train having a period ##EQU2## Alternatively stated, the saturable absorber 20 preferentially passes the largest emission noise spike, which the optical amplifier 14 successively amplifies and narrows as the optical pulse passes through the cavity with a round-trip time of .tau.. Thus, after a relatively short start-up period, the passively mode-locked laser outputs a series of optical pulses having a period .tau. set by the cavity so as to be very stable.
Active mode locking does not require the saturable absorber 20. Instead, the optical amplifier 14 is additionally driven by the RF signal so as to modulate the amplification at the RF frequency f.sub.RF. Such modulation produces phase-locked sidebands spaced on either side of the resonant modes by the modulation frequency f.sub.RF. If the modulation frequency f.sub.RF is chosen to coincide with the mode spacing .DELTA.f, then the RF modulation causes all the resonant modes to be phase-locked. As described above, such a frequency distribution produces an optical pulse train of period .tau..
Passive mode locking produces a pulse train of very short pulses but suffers from cavity-length fluctuations so that the pulse period tends to jitter and satellite pulses appear. Active mode locking prevents such dynamical changes so the pulse period is precisely fixed, but the effect of gain-narrowing produces relatively wide and thus low-intensity pulses. Delfyett et al. combine the two types of mode locking so as to produce very short, high-intensity pulses having a stable period and no satellites. Their hybrid mode-locked laser produced pulses of 6 ps width at a wavelength of 828 nm in a pulsetrain having a repetition rate of 302 MHz. A further optical amplifier and pulse compressor reduced the pulsewidth to 460 fs and raised the peak power to 70 W. Nonetheless, the 6 ps pulsewidth in the mode-locked laser was greater than is theoretically possible. It would thus be advantageous to further reduce the pulsewidth and thus increase the peak power.